The present invention relates to an improved trimmer head for use in flexible line rotary trimming devices used to trim grass, weeds and other vegetation. More particularly, the invention is directed to an improvement in the line loading of both “bump-feed” type trimmer heads such as those disclosed in U.S. Pat. Nos. 4,458,419 and 4,959,904 and the more simple manually operated heads such as that disclosed in U.S. Pat. No. 4,145,809, the contents of said patents being incorporated herein by reference as though fully set forth below.
Trimmer heads used in flexible line rotary trimmers generally carry one or two lengths of flexible nylon cutting line wrapped about an interior spool with the ends of the line or lines projecting outwardly through opposed apertures in the side wall of the trimmer head. The head is threadably mounted on the end of an elongated shaft and rotated at a high velocity by a gas or electric motor so that the ends of the cutting line project radially from the head and sever weeds or other vegetation. When cutting line projecting from the head breaks off or becomes overly worn, it must be severed and fresh line extended from the spool through the line outlet eyelets in the side of the housing. Bump-feed type heads include a line feed-out mechanism which responds to a bump on the ground intentionally applied by the operator to feed out a measured length of fresh cutting line which is typically cut to the desired length by a knife blade projecting from a shield attached to the trimmer above the cutting head and spaced a predetermined distance from the perimeter of the trimmer head housing. Manual heads do not include any such line feed-out mechanism. A fastening nut that holds the housing portion of the trimmer head to the spool must be loosened so that the spool can be separated from the housing and manually rotated relative to the housing to pay out additional cutting line. The spool and housing are then re-secured by the fastening member.
In both bump-feed and manual heads, the length or lengths of cutting line are typically wound onto the spool by hand. As most cutting heads employ two lengths of line projecting from opposed sides of the cutting head, care must be taken during the winding of the spool to avoid crossing or otherwise tangling of the two lines within the spool which interferes with the paying out of fresh line. This is particularly important in bump-feed heads where centrifugal force is utilized to pull the new lengths of line from the spool during use as the head is being bumped against the ground as any line tangle will interfere with the proper feeding of the line. Difficulty in properly loading the line on the spool is the most common complaint of home users of flexible line trimmers. It also is a time consuming task for the professional user.
The early bump-feed mechanisms typically consisted of a dog or friction clutch located between the spool of line and the surrounding housing. By bumping an extension of the spool on the ground, or other fixed object, the friction clutch was temporarily disengaged for a length of time dependant on the duration of the bump. The dog clutch released by the bump then abruptly engaged at the next opportunity to feed out line in segment lengths which were related to the engagement points of the dog clutch. Such dog clutches had outwardly extending ribs which engaged inwardly extending abutment tangs and therefore depended upon a skillful bump when it was desired to deliver only one segment length. However, friction within such devices and overzealous bumping often resulted in two or more line segments being fed out, particularly especially when the device has been in use and the corners on the ribs and tangs became worn such that positive engagement was no longer assured. The unavoidable abrupt operation of the dog clutch caused such wear to occur.
A bump-feed-out mechanism was subsequently developed that automatically fed out a predetermined length of line with each bump, regardless of the duration of the bump, and which did not lose this ability with extended use. That device is disclosed in and is the subject of the incorporated reference, U.S. Pat. No. 4,458,419. As described therein in detail, the improved trimmer head contained a spool holding one or more coils of cutting line and a simplified mechanism that selectively allowed relative movement of the spool with respect to the housing in response to bumping of the head on the ground to pay out measured lengths of line. The simplified pay-out mechanism included a novel spring-loaded cam and cam follower arrangement in which the cam follower included two pair of diametrically opposed and generally inwardly facing abutment surfaces arrayed about the axis of rotation of the trimmer housing. The abutment surfaces were thus spaced 90° apart and were carried by a depending cylindrical wall that circumscribed an interior chamber. The cam member was disposed within the chamber in threaded engagement with the extended lower end of the drive bolt of the trimmer housing and defined two vertically adjacent cams, each cam being of a square configuration and defining four perpendicularly disposed cam surfaces adapted to engage the abutment surfaces on the cam follower. The upper cam was rotationally offset 45° from the lower cam.
In operation, the housing was rotationally driven by the drive bolt through a connection between the upper end of the bolt and the trimmer drive means. The housing and cam member was thus driven by the drive bolt, which in turn drove the cam follower and the spool mounted thereon due to the engagement between the cam surfaces on the cam member and the abutment surfaces on the cam follower. The line carrying spool was disposed about the cylindrical wall of the cam follower and attached thereto via a pair of opposed outwardly projecting studs on the cam follower member that extend into slots formed in the inner portion of the spool. The spool was provided with a bumper at its lower end such that when the bumper was pressed against or bumped on the ground, the housing moved downwardly with respect to the spool against the force of a spring, disengaged the lower cam from the abutment surfaces on the cam follower and allowed the cam member to rotate 45° relative to the cam follower, whereupon the cam surfaces of the upper cam would abut the abutment surfaces on the cam follower. That imparted a similar degree of relative rotation between the spool and the housing. Once the force of the bump was dissipated, the spring loading forced the spool and housing back to their relative positions, which released the cam surfaces on the upper cam from the cam follower abutment surfaces and allowed another 45° of relative rotation of the cam member and cam follower and thus of the spool and housing, for a total of 90° of rotation per bump, which provided the predetermined relative rotation between the housing and spool needed to pay out a desired length of line through the apertures in the trimmer housing. Since the cams interacted with simple, inwardly facing cam follower surfaces formed only on a single level, the release mechanism was deemed relatively economical to manufacture and, due to the large abutment surface areas presented between the cams and cam follower, the device was durable, trouble free and reliable.
Because of early difficulties in molding some of the components of the cutting head disclosed in U.S. Pat. No. 4,458,419, the head became more expensive to manufacture than anticipated. New material developments subsequently reduced the cost of manufacture. In the meantime, however, a similar bump-feed drive mechanism was developed in which large square cams were formed on the upper and lower outer radial surfaces of the spool and the corresponding cam followers were formed by upper and lower portions of the housing which surrounded the spool. Such a head is disclosed in the incorporated reference, U.S. Pat. No. 4,959,904, and is still in production.
Over the years, with increasing competition from offshore manufacturers, it became clear that even with the development of new materials the earlier bump-feed mechanism covered by U.S. Pat. No. 4,459,419 was not as economical to manufacture as earlier believed. It contained several parts, some of which had to be hand assembled. In addition, vibration, the threaded engagement between the cam member and the drive bolt, and the heat generated by the trimmer required the use of a chemical bonding agent having a high melting point to prevent the cam member and cam follower from breaking loose from the drive bolt. Such agents, however, had extremely high break way torques, rendering the threaded connection effectively permanent. As a result, certain components of the head could not be replaced when worn. Thus, that head was significantly modified so as to retain all of the advantages of its predecessor yet utilize fewer component parts and obviate the need for any hand assembly and use of chemical bonding. That modification is the subject of a pending U.S. patent application entitled “Trimmer Head for Use in Flexible Line Rotary Trimmers”, filed Oct. 2, 2003 and identified by Ser. No. 10/677,700, of which this application is a continuation-in-part. The bump-feed and manual heads of the present invention retain all of the advantages of the above-described heads and adds thereto the ability to far more quickly and easily uniformly wind lengths of cutting line onto the spool without materially increasing the cost of production.